This invention relates to separation and feeding of individual sheets from a stack to a point of transfer for use. More particularly, it concerns a sheet feeding method and apparatus particularly suited to handling sheet media of a nature which resists separation of individual sheets from a supply stack, such as plastic sheets which adhere to each other by electrostatic attraction.
Feeding systems for sheet media such as paper have reached a high level of reliability as evidenced by their extensive use in copy machines, high speed printing equipment and the like. Typically, such applications involve the use of calendared paper which facilitates separation of the uppermost sheet in a stack by little more than a friction roller capable of sliding the sheet relative to the remainder of the sheets in the stack. On the other hand, forces tending to resist this basic feeding technique, such as air pressure and mechanical engagement of fibers in paper media, have been recognized and addressed by employing an adhesive sheet lifting arrangement to initially separate the individual sheet to be fed from the remainder f a supply stack. In this respect, U.S. Pat. No. 4,776,575 13 Meyer et al., is representative.
Also, sheet feeders have been devised for handling textile fabrics, which because of their tendency to adhere to each other and also because of their relatively flimsy or non-resilient character, require lifting and separation as an incident to individual sheets being fed from a supply stack. Such arrangements are shown in U.S. Pat. Nos. 3,768,807 --Spangler and 4,231,563 --Boucraut. As taught by these prior patents, textile sheets are separated and fed from a stack by first lifting the trailing edge of the top sheet to be fed using a vacuum pickup and then passing a lifting bar or its equivalent under the lifted trailing edge of the sheet so that the full sheet will be progressively placed against a further vacuum device for lateral feeding.
While the general state of the art relating to sheet separation and feeding is quite developed and sophisticated, it is not known that a reliable system has been devised for handling sheet media of plastic material such as polyester and the like. Plastic sheet media exhibit several characteristics which render known feeding systems virtually ineffective for separation and feeding of individual plastic sheets from a stack. For example, such plastic sheet materials have a very low coefficient of friction, making it difficult to rely on friction to achieve a lateral feeding of one sheet from the remainder of the stack. Also, the relatively non-porous nature of plastic sheet media tends to preclude the presence of any air between the sheets so that atmospheric pressure operates to retain the sheets of a supply stack against individual sheet separation. Finally, and perhaps most significantly, plastic sheet media, being insulative or dielectric, are susceptible to relatively large electrostatic charges by which the individual sheets are attracted to each other under forces which must be overcome if one sheet is to be separated and fed from the stack.
In many applications such as laser photography which require a plastic substrate in sheet form, there is a need for a capability for supplying individual sheets of plastic media to a workstation at relatively high rates of feed. Because of the aforementioned problems with feeding plastic sheet media from a stack, however, no known sheet feeding systems are acceptable for this purpose. Accordingly, there is an acute need for an effective method and apparatus for separating and feeding individual plastic sheets from a supply stack.